Drilling an oil and/or gas well involves creation of a borehole of considerable length, often up to several kilometres vertically and/or horizontally by the time production begins. A drillstring comprises a drill bit at its lower end and lengths of drill pipe that are screwed together. The whole drillstring is turned by a drilling mechanism at the surface, which in turn rotates the bit to extend the borehole. The drilling mechanism is typically a top drive or rotary table, each of which is essentially a heavy flywheel connected to the top of the drillstring.
The drillstring is an extremely slender structure relative to the length of the borehole, and during drilling the string is twisted several turns because of torque-on-bit between about 500 and 10,000 Nm. The drillstring also displays a complicated dynamic behaviour comprising axial, lateral and torsional vibrations. Simultaneous measurements of drilling rotation at the surface and at the bit have revealed that the drillstring often behaves as a torsional pendulum i.e. the top of the drillstring rotates with a constant angular velocity, whereas the drill bit performs a rotation with varying angular velocity comprising a constant part and a superimposed torsional vibration. In extreme cases, the torsional part becomes so large that the bit periodically comes to a complete standstill, during which the drillstring is torqued-up until the bit suddenly rotates again at an angular velocity that is much higher than the angular velocity measured at the surface. This phenomenon is known as stick-slip.
Stick-slip has been studied for more than two decades and it is recognized as a major source of problems, such as excessive bit wear, premature tool failures and poor drilling rate. One reason for this is the high peak speeds occurring during in the slip phase. The high rotation speeds in turn lead to secondary effects like extreme axial and lateral accelerations and forces.
A large number of papers and articles have addressed the stick-slip problem. Many papers focus on detecting stick-slip motion and on controlling the oscillations by operational means, such as adding friction reducers to the mud, changing the rotation speed or the weight on bit. Even though these remedies sometimes help, they are either insufficient or they represent a high extra costs.
A few papers also recommend applying smart control of the top drive to dampen and prevent stick-slip oscillations. In IADC/SPE 18049 it was demonstrated that torque feed-back from a dedicated string torque sensor could effectively cure stick-slip oscillations by adjusting the speed in response to the measured torque variations. In Jansen J. D. et al. “Active Damping of Self-Excited Torsional Vibrations in Oil Well Drillstrings”, 1995, Journal of Sound and Vibrations, 179(4), 647-668, it was suggested that the drawback of this approach is the need for a new and direct measurement of the string torque, which is not already available. U.S. Pat. No. 5,117,926 disclosed that measurement as another type of feedback, based on the motor current (torque) and the speed. This system has been commercially available for many years under the trade mark SOFT TORQUE®. The main disadvantage of this system is that it is a cascade control system using a torque feedback in series with the stiff speed controller. This increases the risk of instabilities at frequencies higher than the stick-slip frequency.
IADC/SPE 28324 entitled “Application of High Sampling Rate Downhole Measurements for Analysis and Cure of Stick-Slip in Drilling” discloses control of a drilling process using driving equipment that includes a PID, a motor, a gear box and rotary table. The PID tries to maintain the desired rotary speed of the drill string and it is suggested that the PID can be adjusted to prevent stick-slip. However, a simulation result shows poor damping of stick-slip oscillations and it is concluded in the paper that PID is too simple a servo-control system to prevent stick-slip.
Our co-pending patent application PCT/GB2008/051144 discloses a method for damping stick-slip oscillations, the maximum damping taking place at or near a first or fundamental (i.e. lowest frequency) stick-slip oscillation mode. In developing the method we have identified a further problem to be addressed when the drill string is extremely long (greater than about 5 km) and the fundamental stick-slip period exceeds about 5 or 6 s. Even though the method of our previous patent application is able to cure the fundamental stick-slip oscillation mode in such strings, as soon as these oscillations are dampened, the second natural mode tends to become unstable and grow in amplitude until full stick-slip is developed at the higher frequency. In certain simulations we have found that this second mode has a natural frequency which is approximately three times higher than the fundamental stick-slip frequency. The higher order stick-slip oscillations are characterised by short period and large amplitude cyclic variations of the drive torque. Simulations show that the bit rotation speed also in this case varies between zero and peak speeds exceeding twice the mean speed.
We have also found through other simulations that the method employed by the aforementioned SOFT TORQUE® system suffers from the same problem. Neither method is able to inhibit both the first and second mode stick-slip oscillations.